1. Field of the Invention
The present invention is directed to a quasi-resonant PWM inverter.
2.Description of the Prior of Art
Some electrical devices such as semiconductor power conversion systems have been miniaturized owing to a trend of the higher switching frequency. On the other hand, it has been recognized that with the production of power inverters there exist problems of ever-increasing switching losses. In order to overcome this problem, several inverter circuits have been proposed; including low loss inverters circuit of the resonant type. Soft switching is achieved in such devices by means of a resonant circuit, allowing the inverter to operate at various MHz frequencies in a small capacity switching power source.
However, An inverter of this type requires additional components to serve a full function, so that there would be some problems when this type of inverter is applied to a large output inverter. Moreover, since it, performs by means of utilizing a resonant type of operation, said inverter necessitates employing of specific switching elements which should have higher ratings than the normal types of inverters. Furthermore, in order to achieve soft switching, the resonant voltage as well as the resonant current are usually fixed at a certain level according to the basis of the maximum out-put condition, so that an identical resonant waveform will be generated even in cases of the low out-put conditions, the conduction loss might increase and the efficiency would be reduced. Moverover, since the resonant frequency is generally constant, the operational frequency in the inverter will be restricted and the degree of freedom for the out-put variability will be also restricted; and accordingly, the control of the inverter becomes complicated. This is a normally recognized trend that has been a common feature that takes place among the resonant type inverters.
In order to, solve the aforementioned problems, a resonant commutated pole inverter was proposed, which will be described by referring to the attached FIGS. 13 and 14. FIG. 13 is a circuit configuration of the resonant commutated pole inverter, while FIG. 14 is a characteristic diagram of said resonant commutated pole inverter.
As seen in FIG. 13, the resonant commutated poke inverter is composed of main switches S1, S2, auxiliary switches S1s, S2s, a reactor for resonance Lr. condensors for resonance having an equal capacity Cr1, Cr2, and condensors Cd whose action is to clamp the middle point of DC power source. With said resonant commutated pole inverter, the middle point voltage, E/2, of the DC power source is divided by the condensors Cd.
As in the characteristic diagram as seen in FIG. 14, the waveforms at certain locations are shown when the diode D2 in the main switch S2 is commutated to the main switch S1. Suppose that the output current, Io, is positive and said current, Io, is kept constant during a period of time that has been commutated; when the auxiliary switch S1s turned on under a condition in which the output current is being circulated to the diode D2, the current Ir flowing in the reactor Lr for resonance will be increasing linearly. With this condition, when Ir=Io, the current flowing in the diode D2 will become nil and the current flowing in a transistor in the main switch. S2 will become Ir-Io.
At a moment when the current Ir-Io, reaches the preset current value, Ibt, and if the transistor of the main switch S2 is turning off, then the resonant operation will start, the voltage, Vo, of the condenser Cr2 for the resonance will increase as a result of said resonant operation, Once the voltage reaches the power source voltage, E, the diode D1 in the main switch S1 Will be on. At this moment, the current flowing in the diode D1 becomes Ir-Io and will decrease linearly. By providing an ON-signal to the transistor in the main switch S1 during the time of conduction of the diode D1, lossless switching In the main switch S1 will be achieved, When Ir=0, the output current will be supplied through the main switch S1 and this will lead to complete the commutating operation.
In the previously described case, the current, Ibt turning off the transistor in the main switch S2 compensates the losses that are present in the resonance circuit; including the voltage drop loss in switches or diodes, copper loss or iron loss in the reactor Lr for resonance, and loss that occurs to internal resistance in condensors Cr1, Cr2 for resonance. Moreover, said current is continuously supplied in order to raise the voltage of the condenser Cr2 for resonance up to the power source voltage E. If the current Ibt is less than the optimum value, the level of the voltage Vo of the condenser Cr2 for resonance might not be able to be raised to the power source voltage, E.
As a result, the main switch S1 will be turned-on while electric charge is remaining in the condensor Cr1 which is connected in parallel to said main switch S1. Hence a large loss will be expected to take place inside the main switch S1, and this might lead to a problem such, that element will be likely subjected to a thermal failure.
In contrast, when the current Ibt exceeds the optimum value the resonant current amplitude will become large, so that the conduction loss increases and a certain time required to the commutation operation, in other words dead time, changes, and the output variable range can not be properly secured.
Thus the control of the current Ibt should be done appropriately for said resonant commutated pole inverter and when it is applied to inverters that have more than a medium output power level including IGBT, BJT, GTO or the like, influence of delay time of switching is more, likely to become a serious problem. As has been already explained, since the current Ibt is controlled by a timing of turning-off the main switch S2, a complicated control system including a current feedback mode is necessitated so that it will act to compensate for the switching delay time and to determine the current, Ibt, precisely. Moreover, since the switching elements with relatively large voltage ratings will generally exhibit high voltage drop, the loss inside the resonant circuit will increase and in turn the required current Ibt will also raise and the influence of the delay time will become remarkable.